In recent years, more emphasis has been placed on laminating adhesives prepared from water-borne systems. These systems are generally based on acrylic-vinyl ester emulsions (latexes) which are prepared in emulsion form using conventional emulsion polymerization techniques. There are, however, problems incident to these adhesives, some of which result from the presence of residual surfactant in the compositions. In formulating conventional adhesive compositions suitable for laminating flexible films it is generally necessary to employ surfactants in at least two stages of the preparation. First, surfactants are conventionally used in the emulsion polymerization as protective stabilizers for the polymerized emulsion particles. Secondly, since many of the films which are commonly coated with these laminating adhesives, e.g. polyethylene, polypropylene and silicone coated release liner, are difficult to wet due to their low surface energies, it is generally necessary to post-add additional or other surfactant to the adhesive to facilitate wetting the film or other substrate prior to forming the laminate.
The presence of these conventional surfactants in the cured adhesive films has been found to be detrimental to the laminations for a variety of reasons. In particular, when the adhesive is to be used to form laminates wherein at least one of the surfaces is a printed surface, the presence of the surfactant often leads to discoloration or blurring of the ink. This is recognizably a problem in applications such as overlaminating of books or printed labels where the purpose of the outer surfacing film is to preserve the integrity of the printed surface. It is also known that the presence of surfactant reduces the wet bond strength and water resistance of the laminate.
In order to avoid post-added surfactants, it is desirable to dilute the adhesive compositions with sufficient alcohol to provide adequate wetting. Most laminating adhesive compositions, however, do not have adequate tolerance for addition of the total quantity of alcohol required and therefore post-added surfactant or post-added surfactant plus low levels of alcohol are the most commonly employed methods of lowering their surface tension.
It is an object of the present invention to provide an emulsion adhesive wherein substantially no free surfactant is present in the final adhesive composition.
It is a further object of the invention to provide an emulsion adhesive composition characterized by superior alcohol tolerance so as to avoid the necessity for post-adding surfactant to improve wetting properties.
These and other objects will be obtained from the adhesives of the present invention.